Hybrid Physical-Chemical Vapor Deposition

Hybrid physical-chemical vapor deposition (HPCVD) is a thin-film deposition technique that combines physical vapor deposition

Physical vapor deposition

Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...

(PVD) with chemical vapor deposition

Chemical vapor deposition

Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...

(CVD). For the instance of magnesium diboride

Magnesium diboride

Magnesium diboride is a simple ionic binary compound that has proven to be an inexpensive and useful superconducting material.Its superconductivity was announced in the journal Nature in March 2001. Its critical temperature of is the highest amongst conventional superconductors...

(MgB₂) thin film growth, HPCVD process uses diborane

Diborane

Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...

(B₂H₆) as the boron precursor

Precursor (chemistry)

In chemistry, a precursor is a compound that participates in the chemical reaction that produces another compound. In biochemistry, the term "precursor" is used more specifically to refer to a chemical compound preceding another in a metabolic pathway....

gas, but unlike conventional CVD, which only uses gaseous sources, heated bulk magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

pellets (99.95% pure) are used as the Mg source in the deposition process. Since the process involves chemical decomposition

Chemical decomposition

Chemical decomposition, analysis or breakdown is the separation of a chemical compound into elements or simpler compounds. It is sometimes defined as the exact opposite of a chemical synthesis. Chemical decomposition is often an undesired chemical reaction...

of precursor gas and physical evaporation

Evaporation (deposition)

Evaporation is a common method of thin film deposition. The source material is evaporated in a vacuum. The vacuum allows vapor particles to travel directly to the target object , where they condense back to a solid state...

of metal bulk, it is named as hybrid physical-chemical vapor deposition.

System configuration

The HPCVD system usually consists of a water-cooled reactor chamber, gas inlet and flow control system, pressure maintenance system, temperature control system and gas exhaust and cleaning system.

The main difference between HPCVD and other CVD systems is in the heating unit. For HPCVD, both substrate and solid metal source are heated up by the heating module. The conventional HPCVD system usually has only one heater. The substrate and solid metal source sit on the same susceptor

Susceptor

A susceptor is a material used for its ability to absorb electromagnetic energy and convert it to heat . This energy is typically radiofrequency or microwave radiation used in industrial heating processes, and also occasionally in microwave cooking...

and are heated up inductively

Induction heating

Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...

or resistively

Joule heating

Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...

at the same time. Above certain temperature, the bulk metal source melts and generates a high vapor pressure

Vapor pressure

Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

in the vicinity of the substrate. Then the precursor

Precursor (chemistry)

gas is introduced into the chamber and decomposes around the substrate at high temperature. The atoms from the decomposed precursor gas react with the metal vapor, forming thin films on the substrate. The deposition ends when the precursor

Precursor (chemistry)

gas is switched off. The main drawback of single heater setup is the metal source temperature and the substrate temperature cannot be controlled independently. Whenever the substrate temperature is changed, the metal vapor pressure

Vapor pressure

changes as well, limiting the ranges of the growth parameters. In the two-heater HPCVD arrangement, the metal source and substrate are heated up by two separate heaters. Thus it can provide more flexible control of growth parameters.

Magnesium diboride thin films by HPCVD

HPCVD has been the most effective technique for depositing magnesium diboride

Magnesium diboride

(MgB₂) thin films. Other MgB₂ deposition technologies either have a reduced superconducting transition temperature and poor crystallinity

Crystallinity

Crystallinity refers to the degree of structural order in a solid. In a crystal, the atoms or molecules are arranged in a regular, periodic manner. The degree of crystallinity has a big influence on hardness, density, transparency and diffusion. In a gas, the relative positions of the atoms or...

, or require ex situ annealing in Mg vapor. The surfaces of these MgB₂ films are rough and non-stoichiometric. Instead, HPCVD system can grow high-quality in situ pure MgB₂ films with smooth surfaces, which are required to make reproducible uniform Josephson junctions

Josephson effect

The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...

, the fundamental element of superconducting circuits.

Principle

From the theoretical phase diagram

Phase diagram

A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...

of Mg-B system, a high Mg vapor pressure

Vapor pressure

is required for the thermodynamic phase stability of MgB₂ at elevated temperature. MgB₂ is a line compound and as long as the Mg/B ratio is above the stoichiometric 1:2, any extra Mg at elevated temperature will be in the gas phase and be evacuated. Also, once MgB₂ is formed, it has to overcome a significant kinetic barrier to thermally decompose. So one does not have to be overly concerned about maintaining a high Mg vapor pressure

Vapor pressure

during the cooling stage of the MgB₂ film deposition.

Pure films

During the growth process of magnesium diboride thin films by HPCVD, the carrier gas is purified hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

gas H₂ at a pressure of about 100 Torr

Torr

The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

. This H₂ environment prevents oxidation during the deposition. Bulk pure Mg pieces are placed next to the substrate on the top of the susceptor

Susceptor

. When the susceptor

Susceptor

is heated to about 650°C, pure Mg pieces are also heated, which generates a high Mg vapor pressure

Vapor pressure

in the vicinity of the substrate. Diborane

Diborane

(B₂H₆) is used as the boron

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

source. MgB₂ films starts to grow when the boron

Boron

precursor

Precursor (chemistry)

gas B₂H₆ is introduced into the reactor chamber. The growth rate of the MgB₂ film is controlled by the flow rate of B₂H₆/H₂ mixture. The film growth stops when the boron

Boron

precursor

Precursor (chemistry)

gas is switched off.

Carbon-alloyed films

To improve the performance of superconducting magnesium diboride

Magnesium diboride

thin films in magnetic field, it is desirable to dope

Doping (semiconductor)

In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...

impurities into the films. The HPCVD technique is also an efficient method to grow carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-doped or carbon

Carbon

-alloyed MgB₂ thin films. The carbon-alloyed MgB₂ films can be grown in the same way as the pure MgB₂ films deposition process described above except adding a metalorganic magnesium precursor

Precursor (chemistry)

, bis(methylcyclopentadienyl)magnesium

Sandwich compound

In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives...

precursor, into the carrier gas. The carbon-alloyed MgB₂ thin films by HPCVD exhibit extraordinarily high upper critical field (H_c2). H_c2 over 60 T

Tesla (unit)

The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

at low temperatures is observed when the magnetic field is parallel to the ab-plane.

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